For communication between sensors and control devices, the CAN bus system has found widespread acceptance. In the case of the CAN bus system, messages are transmitted using the CAN protocol, as described in the CAN specification in ISO11898. As the number of intelligent sensors increases and the control devices are networked more intensively in the vehicle, the number of subscriber stations on the CAN bus and the volume of data on the CAN bus are increasing more and more.
DE10 000 305 A1 describes the CAN (Controller Area Network) and an extension of the CAN which is referred to as TTCAN (Time Trigger CAN). The media access control method used in the CAN is based on bitwise arbitration.
In the case of CAN, the bitwise arbitration is carried out on the basis of a leading identifier inside the message to be transmitted via the bus.
As already described in DE 10 2012 200 997, a plurality of subscriber stations can simultaneously transmit data via the bus system during the bitwise arbitration without thereby interfering with the transmission of data.
Recently, technologies have been proposed, for example CAN-FD in which messages are transmitted according to the specification “CAN with Flexible Data-Rate, Specification Version 1.0” (source http://www.semiconductors.bosch.de), etc. In the case of such technologies, the maximum possible data rate is increased above a value of 1 Mbit/s by using higher clocking in the region of the data fields. CAN-FD makes it possible to increase the data rate for systems in which the data rate was previously limited by the bus length of the systems.
The bus topology plays a significant role in the reflection-free and therefore fast transmission of data. Ideally, there are only two CAN subscriber stations on the CAN bus. In this case, the bus ends can be ideally terminated and line reflections can be avoided. In order to save transmission cables and to actually be able to use the advantages of the CAN protocol, it is desirable in practice, however, to connect as many CAN subscriber stations as possible to a bus.
However, the problem is that reflections are produced in the transmission of data at each branch of the data lines. These reflections are superimposed on the original signals and interfere with the reception by the receivers. The greater the reflections, the slower the data rate must be selected in order to still be able to reliably transmit the signal.
In order to ensure reliable transmission, the CAN protocol provides error handling. According to error handling, each CAN subscriber station checks all signals on the CAN bus and terminates the transmission with an error frame if an error is identified. Even the CAN subscriber stations which are not involved in communication, because they do not further process the signals transmitted via the CAN bus for example, intervene in the communication between the transmitter and the receiver.
The interference signals occurring on the CAN bus have a different effect on the CAN subscriber stations. The influence of the interference is greater, the more strongly the useful signal is attenuated in comparison with the interference. It is generally the case that the shorter the line between the interference source and the receiver and the longer the line between the transmitter and the receiver, the worse the signal quality. As a result, the situation may occur in which the receiver could receive the signal without any errors, whereas a CAN subscriber station not involved in communication identifies an error and destroys the signal by an error message. This results in the transmitter having to transmit the signal again, which unnecessarily increases the bus load on the CAN bus and unnecessarily slows down the transmission of data.
In order to be able to reliably detect an erroneous signal, the evaluation of the CRC signal is sufficient in most cases.
In addition, it is possible to achieve an even higher data rate by transmitting the data inside the CAN frame in a similar manner to data transmission protocols, for example Ethernet. However, such protocols currently cannot be readily used.
Another problem is that the times at which one bit is intended to be sampled is set for each individual subscriber station when designing a CAN network or bus system. This setting is also referred to as bit timing. Depending on the subscriber station from which the signal is received, other times are optimal for error-free reception. However, the times cannot be varied on the basis of the transmitting subscriber station. Therefore, a compromise for the best times must be found when designing a CAN network or bus system by taking into account all subscriber stations. In particular, when designing a network having a plurality of CAN-FD subscribers, it is difficult to set the times in such a manner that all subscribers can receive the signal without errors. If a suitable time cannot be found, the data rate of the bus must be reduced as a solution to this.